The present invention relates to an electric power steering system of an automobile; and, more particularly, to an electric power steering system with an anti-lock brake system (ABS) capable of improving steering characteristics of a vehicle when the vehicle has a sharp braking on a road surface where left and right driving wheels have different road surface frictional forces.
In general, in a reaction type electric power steering system, a steering effort required for a vehicle operator to steer a vehicle may be increased by increasing a pressure to a reaction member in proportion to a speed of the vehicle. Accordingly, the vehicle operator can operate a steering wheel with a relatively small steering effort when the vehicle is traveling at a low speed. However, when the vehicle is running at a high speed, a steering of the vehicle involves a comparatively large amount of steering effort of the vehicle operator.
An anti-lock brake system electronic control unit (ABS ECU) properly controls a brake pressure of driving wheels during a sharp braking to thereby prevent a risk that might be caused by a locking phenomenon of driving wheels.
However, when a frictional force between a left driving wheel and the road surface is different from that between a right driving wheel and the road surface during the sharp braking, the vehicle may be revolved so that the operator may hastily steer the vehicle. In such case, the vehicle is often over-steered, causing the vehicle operator to be at risk of a traffic accident.
In the conventional reaction type electric power steering system, when the vehicle operator suddenly brakes the vehicle and the ABS ECU generates an ABS operational signal, the information on the speed of the vehicle is fixed as one high value and accordingly the pressure applied to the reaction member is increased, resulting in an increase of the steering effort as well. Thus, a steering response to a specific steering action becomes smaller than that of a normal case and a stability of a vehicle behavior is secured.
However, this conventional art has certain drawbacks in that it is difficult to perform a minute tuning and steering effort cannot be infinitely increased on a slippery road surface. In order to overcome these drawbacks, an additional steering gear with a wide control range has been required. Further, there exists a high risk that the steering wheel is suddenly revolved due to a kick back of the driving wheels during the sharp braking, thereby making the confused vehicle operator more frightened.
It is, therefore, an object of the present invention to provide a motor control device for use in an electric power steering system capable of reducing unnecessary movements of a steering wheel caused by road interface frictions by controlling a driving current of an electric power steering motor depending on a braking power difference between the left and the right driving wheels of a vehicle when an anti-lock brake system (ABS) is operated.
In accordance with the present invention, there is provided an electric power steering system, including:
a torque detection unit for sensing a manual steering torque applied to a steering wheel of a vehicle and detecting a braking power difference torque representing a difference in braking powers between left and right driving wheels of the vehicle;
an anti-lock brake system (ABS) electronic control unit for outputting an ABS operational signal which provides information on a sharp braking of the vehicle;
an electric power steering control unit for generating a motor current in response to the manual steering torque provided from the torque detection unit and, if the ABS operational signal is provided, outputting a modulated motor current obtained by increasing or decreasing the motor current based on the braking power difference torque; and
an electric power steering motor, which operates based on the modulated motor current provided from the electric power steering control unit, for reducing unnecessary movements of the steering wheel caused by a difference in road surface frictional forces of left and right driving wheels of the vehicle.